This is one of the dirtiest moves in a bar fight! Ussually done when the opponent is both unaware of the assanign onslaught about to ensue and sitting at the bar with his back turned.
The McCreedy's pub jig is performed as thus: your opponent is sitting at the bar of his favored pub, when two of your friends walk up on either side of him and pretend to order drinks. At this point you (if so inclinded) get a running start and slam ones elbow into the base of the back of their head and drive it down to the bar as you and one of you friends on either side help to hold him down as the third of your party climbs onto the bar an begins to forcefully stomp the back of his head into the bar.